1. Field of the Invention
The present invention relates, in a broad sense, to a technical field of phosphors other than silicate phosphors. More specifically, it relates to a phosphor-coating composition based on such phosphors, and a discharge lamp using the same.
Silicate phosphors, no matter when used singly or in combination, comprised mainly of silicate (50% by weight or more), whereas the present invention relates to a phosphor whose major component is other than silicate (i.e., even when the silicate phosphor is present, its content is less than 50% by weight). Hereinafter, such phosphors containing silicate phosphors, if present, in an amount of less than 50% by weight will be referred to simply as phosphors.
2. Description of the Related Art
Phosphors have been commonly utilized for a discharge lamp, for instance, a fluorescent lamp. They have been used not only singly, but also in mixture. Such a mixture includes as a so-called three-component type phosphor composition containing a red luminous phosphor, a blue luminous phosphor and a green luminous phosphor. Recently, even a five-component type phosphor composition having an improved color rendering property has been commercially available. To reiterate, phosphors of the type used in the invention are clearly distinguished in terms of the major component from silicate phosphors whose major component is silicate.
In general, a discharge lamp, e.g., a fluorescent lamp is prepared in such a manner that phosphors are suspended in a highly viscous solution (vehicle) prepared by dissolving a binder in a solvent, to prepare a phosphor suspension (phosphor-coating liquid). The phosphor-coating liquid is coated on an inner wall of a bulb, and the coated material is baked, obtaining a fluorescent lamp. The phosphor-coating liquid may be roughly classified into two types depending on the kind of the solvent used: one is organic type using an organic solvent and the other is aqueous type using water as a solvent.
Meanwhile, in order to prevent the phosphor layer from peeling off the inner wall of the bulb, an adhesive material is usually used together. Such an adhesive material may be added to the phosphor-coating liquid, or alternatively may be previously attached to the surfaces of phosphor particles.
As adhesive materials added to the phosphor-coating liquid, there are known alkaline earth borates such as calcium barium borate, calcium pyrophosphate, and aluminum oxide. There are also known a first glass prepared from calcium pyrophosphate and calcium tetraborate, and a water-resistant glass composition having a low melting point, expressed by Ln.sub.2 O.sub.3 --MnO--B.sub.2 O.sub.3, where Ln is Y or a lanthanoid, M is Mg, Ca, Sr, Ba, and/or Zn).
These conventional adhesive materials, however, often can not produce desired adhesive effects unless they are added in a relatively large amount (e.g., at least 2% by weight or more) relative to the phosphor to be coated on the inner wall of the bulb. Further, they tend to lower not only the initial luminous flux of the fluorescent lamp, but also the luminous flux maintenance of the phosphor layer during lightening.